The present invention relates to a method for producing a quartz glass tube by feeding a cylinder of quartz glass rotating about a rotational axis continuously to a heating zone, softening the cylinder therein in portions starting from one end, and drawing the softened portion over a drill head of a drill body arranged to be coaxial to the rotational axis, thereby forming the portion into the quartz glass tube.
Furthermore, the present invention relates to a drill body for forming a cylinder of quartz glass into a quartz glass tube, the drill body comprising a drill head which penetrates into the cylinder to be formed and which is held by a drill shaft.
A method and an apparatus of this type are described in JP-A 3-247525. In the method described therein, a solid cylinder of quartz glass is formed into a hollow cylinder of quartz glass by the measures that the solid cylinder of quartz glass while rotating about its longitudinal axis is continuously fed to a furnace, starting at one end, the solid cylinder is softened therein in zones and the softened portion is pressed against a fixed drill body arranged within the rotational axis and is drawn thereover. The drill body comprises a conically shaped drill head which is held on a long shaft.
During the forming process considerable forces act on the drill head. The rotating glass material of the solid quartz-glass cylinder can produce static friction forces acting on the drill head at one side, said forces deflecting the drill head laterally out of the axis of rotation. Moreover, the drill shaft may easily bend because of its length, thereby tilting the drill head out of the horizontal, so that the upper part of the drill head face is no longer in engagement with the quartz glass. In the known method these effects yield a bore that is not straight enough and a bore diameter that is insufficient in its dimensional stability. Radial grooves are produced in the bore surface by a glass material resting on the drill head at one side.
It is therefore the object of the present invention to indicate a method for forming a cylinder of quartz glass into a hollow cylinder of quartz glass having an inner bore that is as flawless as possible and is straight and dimensionally stable, and to provide a drill body therefor.
As for the method, this object starting from the above-mentioned method is achieved according to the invention in that a drill body is used with a drill head having a contact surface of a convex curvature facing the cylinder.
In the method according to the invention, use is made of a drill head which has a contact surface of a convex curvature. During the forming process the contact surface is always in planar engagement with the softened glass material of the cylinder to be formed. To this end the contact surface has a convex curvature. As a result, even upon a deflection of the drill headxe2x80x94for instance because of a deflection of the drill rod on which the drill head is heldxe2x80x94the planar contact with the softened glass material is maintained. This guarantees that, on the circumference of the drill head, friction forces that are always of about the same magnitude are active in rotationally symmetrical fashion around the axis of rotation, thereby preventing the formation of unilaterally acting static friction forces that might deflect the drill head even further. Thanks to the continuous planar contact, the quartz glass displaced in the inner bore of the quartz-glass tube is smoothly molded off on the edge zone of the convex contact surface, leaving no defects in the bore surface. Grooves are thereby avoided. What is essential is that the contact surface has a convex curvature extending in symmetry with the axis of rotation. Ideally, the curvature is spherical; this means that it forms part of a spherical surface. Deviations from the spherical shape towards the surface of an ellipsoid or paraboloid are possible.
The cylinder to be formed is in general a solid cylinder. However, the method according to the invention is also suited for enlarging the inner bore of a hollow cylinder.
Preferably, use is made of a drill head with a spherically shaped contact surface. Upon deflection of the drill head from the axis of rotation, the spherical contact surface will always remain over its whole surface in engagement with the softened quartz-glass material, independently of the direction of deflection, so that frictional forces that are always of the same magnitude in terms of rotational symmetry act on the circumference of the drill head and the quartz glass displaced in the inner bore is smoothly molded off on the edge zone of the ball or sphere.
It has turned out to be of particular advantage when the contact surface is provided with recesses. Gases entrapped in the softened quartz glass or formed during the forming process can be discharged through the recesses or at least be distributed in a more uniform manner.
Preferably, recesses which extend in concentric fashion relative to the axis of rotation are provided on the drill head. These enable the gases to expand and permit a uniform distribution of the gases around the circumference of the drill head. This avoids a situation where the drill head partially xe2x80x9cfloatsxe2x80x9d because of gas bubbles in the softened quartz glass, thereby losing the desired full-surface contact with the glass material. Thanks to the uniform distribution of the gases the static friction forces that are uniformly distributed around the circumference of the drill head are maintained so that straightness of the bore as well as dimensional stability of the bore diameter are ensured.
As for the drill body, the above-mentioned object starting from the drill body of the above-mentioned type is achieved according to the invention in that the drill head has a contact surface of a convex curvature.
The drill body according to the invention has a contact surface of a convex curvature. As a consequence, during the forming process the contact surface is always in planar engagement with the softened glass material of the cylinder head to be formed. As a result, even upon a deflection of the drill headxe2x80x94for instance because of a deflection of the drill rod on which the drill head is heldxe2x80x94the planar contact with the softened glass material is maintained. This guarantees that, on the circumference of the drill head, friction forces that are always of about the same magnitude are active in rotationally symmetrical fashion around the axis of rotation, thereby preventing the formation of unilaterally acting static friction forces that might deflect the drill head even further. Thanks to the continuous planar contact, the quartz glass which is displaced in the inner bore of the quartz-glass tube is smoothly molded off on the edge zone of the convex contact surface, leaving no defects in the bore surface. Grooves are thereby avoided. What is essential is that the contact surface has a convex curvature extending in symmetry with the axis of rotation. Ideally, the curvature is spherical; this means that it forms part of a spherical surface. Deviations from the spherical shape towards the surface of an ellipsoid or paraboloid are possible.
In a preferred embodiment of the drill body according to the invention, the drill head comprises a spherically shaped contact surface. Upon deflection of the drill head from the axis of rotation, the spherical contact surface will always remain over its whole surface in engagement with the softened quartz-glass material, independently of the direction of deflection, so that frictional forces that are always of the same magnitude in terms of rotational symmetry act on the circumference of the drill head and the quartz glass displaced in the inner bore is smoothly molded off on the edge zone of the ball or sphere.
It has turned out to be of particular advantage when the contact surface is provided with recesses. Gases entrapped in the softened quartz glass or formed during the forming process can be discharged through the recesses or at least be distributed in a more uniform manner.
Preferably, the recesses extend in concentric fashion relative to the axis of rotation. Recesses formed in this way enable the entrapped gases to expand and permit their uniform distribution around the circumference of the drill head. This avoids a situation where the drill head partially xe2x80x9cfloatsxe2x80x9d because of gas bubbles within the softened quartz glass, thereby losing the desired full-surface contact with the glass material. Thanks to the uniform distribution of the gases, the static friction forces that are uniformly distributed around the circumference of the drill head are maintained so that straightness of the bore as well as dimensional stability of the bore diameter are ensured.
In a particularly preferred embodiment of the drill body according to the invention, the drill head is provided with a spherical surface and concentrically recessed steps. The number of the concentrically recessed steps depends on the diameter of the inner bore to be produced and on the amount of gases formed during the forming process.
It has turned out to be useful when a drill head is provided with at least one cooling channel. A coolant for discharging heat from the area of the drill head can be passed through the cooling channel during the forming process.